I have a fairly bad-ass 93" iron sportster (street) that is at the end of it's HP potential in current form. It is 3.5' X 4-13/16", 12:5 to 1, Red-shift 550 grind, Super 'D' w/ two t-jets, etc. Zippers did most of the engine/trans.

My biggest limitation I think are the cylinder heads. They have the big XR valves, dual plugs, port work, etc. The ports can only be enlarged slightly due to the thin-wall design and do not flow nearly enough for the combination I have.

What I really would like to find is a set of Thunderheads in any condition to go to the next level.

For those who don't know these are basically aluminum copies of the stock iron head with better valve angles, combustion chamber and port size. They were manufactured by a small company in Texas back in the late 60's to 70's
There were quite a few sets sold over the years and I missed out on a set a while back.

So if anyone knows a better solution or the where-abouts of a set of these heads please let me know. I will even pay a substantial finders fee

Speed is just a question of money....................how fast do you want to spend?

This subject comes up about once a year and I have to laugh!!!!!,the last bike I saw in an ad to have a set of THUNDERHEADS claimed over a 115 hp;BULLSHIT!!!. rOBISON iNDUSTRIES WERE BASED IN ca., and the Thunderheads were a piss-poor copy of XR-750 heads;the limiting factor being the single carb. Anyone who knows anything about HP Harleys KNOWS THE ONLY WAY FOR HIGH hp IS TO RUN TWO CARBS ON SEPARATE INDUICTION TRACTS. Owning 2 XR1000's and a custom based XR 1000,I know what I'm talking about. "A LITTLE KNOWLEDGE CAN BE A DANGEROUS THING" BREWSKI

I would agree with Brewski , forget about those heads , the company went bust owing money to pre-ordered customers (not me)
Why not go for a pair of Shovelheads (as in shovester) they must be very available over there . Trock Cycle made cylinders to suit the Sporty lower end and accomadate the shovel top , call them on 312 6834010 and also Carls Speed shop carried out this conversion , he was at 9411 Santa Fe Springs rd , Santa Fe Springs
Ca .90670 .both these contacts are over 20 years old so I don't know if they are still around.
You would probably need to get the heads ported , maybe by Branch ,I'v seen several bikes with these heads converted to single carb manifolds , one facing forward and the front going back
with this set-up you would still have piston availability and its a tried and tested conversion , plus its all Harley . Hope this helps out.

I have to disagree,ALAN SPUTHE offered aluminum heads for the XL in the early '80's,I have the original magazine articles;these were twin carb heads for the 1000cc bikes. These heads are still on Sputhe's website the last time I checked,he later went on to build 80" XR based engines."A LITTLE KNOWLEDGE CAN BE A DANGEROUS THING" BREWSKI One of his 1400cc bikes was for sale @ HANFORD a couple of years ago.

yes i know alan very well. he is in grass valley cali. i have put two sets his heads on motors. some time ago. kinda hard to get. i have friend who has set will check see if he wants to sell. alans is for 2 carbs. like xr 750. ever thik of xr 1000 heads? i would check ur bolt pattern. i worked for tom sifton at one time. we talked about putting bsa gold star heads on a harley. at one time 1959 harley made 8 sets of alum heads {single carb} for there race dept. i only know of 2 sets left. i have 1 and friend of mine in fresno has other one. ever check with branch? seems like u can get ur cast iron heads to flow quite well. i have built a few strokers. problem with thunder heads is pistons have dome like xr 750. so u need there pistons. and rocker boxes. i tryed to get stroker pistons for thunder heads. best i could do is have them made j&e. but piston pin would go into oil ring.

XLR - I don't think I would go for XR heads from Sputhe or Harley. I don't have room for the exhaust on the left side and rearward-facing carbs on the right.

I'd like to see the race heads you're talking about. As far as the thuinder heads yes, it would have to be the complete deal and I could have pistons made to match the combustion chamber.

As far as my heads go they have been modified as far as you can safely take them. Main problem is the port size will not flow enough air to really let this thing breathe. The flow % numbers go south past 4500.

I left a message on your voice mail. Let me know.

Thanks!!

Speed is just a question of money....................how fast do you want to spend?

Haven't done it, but IMHO the geometry can be fixed without too much work. Sealing the tubes will be a little more work. The overhang can be potentially reduced to bring the levers inboard and closer to the tappets.

Zippers told me ( a while ago) that it was not worth doing. If fact the ports are TOO big on a shovel head even with a large motor. Before some of the aftermarket heads came out they were actualy welding the floor of the intake and exhaust to increase flow. Somewhat like a "D" port

The other issue is the added height to the engine. I have a stock, but extensively modified '77 cradle and the engine is already .600" taller and just barely fits.

Speed is just a question of money....................how fast do you want to spend?

Thunderheads use modified XL rocker arms,XL strokers have the wrist pin boss moved up into the oil ring land area. I repeat what I said earlier Thunderheads WERE a piss poor copy of the Round port (early) XR heads. Thunderheads NEVER delivered the HP potential that was advertise,if you really wanted to go with aluminum heads use either XR750 heads or STD dual carb heads,LAWWILL uses these on his STREET TRACKER bikes. Gary JOHNSON of drag racing fame modified a few sets of XR750 heads to use EVO XL cylinders. Like I always say"A LITTLE KNOWLEDGE CAN BE A DANGEROUS THING" brewski

BREWSKI wrote:This subject comes up about once a year and I have to laugh!!!!!,the last bike I saw in an ad to have a set of THUNDERHEADS claimed over a 115 hp;BULLSHIT!!!. rOBISON iNDUSTRIES WERE BASED IN ca., and the Thunderheads were a piss-poor copy of XR-750 heads;the limiting factor being the single carb. Anyone who knows anything about HP Harleys KNOWS THE ONLY WAY FOR HIGH hp IS TO RUN TWO CARBS ON SEPARATE INDUICTION TRACTS. Owning 2 XR1000's and a custom based XR 1000,I know what I'm talking about. "A LITTLE KNOWLEDGE CAN BE A DANGEROUS THING" BREWSKI

Dec. 1979 Motorcyclist magazine. Stock IH dyno test gives 45.56hp and 43.61 ft-lbs at the rear wheel. Add Thunderheads, HD Cafe Racer pipes and 36 mm Dell'Orto Carb and dyno got 70.31 hp and 65.1 ft-lbs. The stock pipes were very restrictive but the CR pipes added no power, they just boosted mid-range torque to 53.04 ft-lbs. By the way, the 36 mm carb produced better power than a 38 mm Dell'Orto.

With one carb, this is identical to what a stock XR1000 produced with two. Yes with Gerry Branch porting, hotter cams and re-jetting the XR-1000 could produce about 100 hp. The same changes would benefit the Thunderhead equipped engine as well. And yes you can get more power with dual carbs, but a 24 horsepower bolt on is not a "piss poor copy" by any stretch of the imagination.

What I want to do is convert the thunder heads into dual intake / custom ports. Since these are aluminum you could do just about modification you'd want.

Thses are the only aftermarket head ever offered for the iron sportster. So I want as set. Any leads?

UI have extensive fabrication /welding skills and have access to one of the best machine shops in the country. They are a complete facility with a flowbench.

Well this comes back 8 years after the original post. Thunderheads on a 93" engine is a no go. The valves are way too small for a 90 plus inch engine. These heads were designed for 1000 cc's, not 1500. There is no room for bigger valves due to the shape and size of the combustion chamber. XR 750 heads would be worse, as they are even smaller, and designed for an engine of less than half the displacement of a 93 incher. The best option is to get your iron heads done by someone like Dan Baisley. In an aftermarkey head, Alan Sputhe's alloy heads were designed for an 80 inch motor. They easily produced 100 hp with his barrels and cylinders, and they are set up for dual carbs, and right side dual exhaust if I remember correctly. No XR or Thunderheads are going to do the job. Sputhe heads may be easier to find as well. If you do find a use for Thunderheads on a smaller motor, there are two three you need to know that many guys apparently don't. You need to degree your flywheels and make new timing marks at 25 and 28 degrees, and you need to get or make a small bushing to modify the mechanical advance unit so you only get 25 degrees total advance instead of 40. The rocker arms have to be cut and welded to different angles if you don't get them with the heads, and absolute maximum lift at the cam lobes is .450. Do not try to exceed. Andrews X cams do the job very well.

I just found this forum, I hope some body still paying attention to this Thunder head stuff.I bought a set in late 1980 after my 79 sporty came apart with 4000 miles on it. The Crank pin nut came off because the factory decided to use locktite instead of the screw on locking thingamajig they used for years, luckily didnt damage the cases.Prior to falling apart my stock 79 sporty ( pipes & lectron carb ) would outrun my 74 CB 750 with similar modifications by 4 - 6 bike lengths up to 100mph then the Honda would pull ahead. I had 4 or 5 buddies each with sportys & mine was the quickest - I don't know why.After installing Thunderheads & Thundercams made by Sifton, back in the 80's this was one of the quickest HDs around, Easily beat Honda Triumph, Norton, & depending on the rider I would pull a Kaw 1000, It would barely run on pump gas but ran like a bear on 110. I'm guessing a gain of 30+HP,It would pull to 7500 RPM before you knew it , probably why I've had to rebuild the bottom end 5 or 6 times. Recently (2 or 3 years) ago rebuilt it again S&S rods & crank pin, and true spin balance job. I probably only have 200 miles since, I can't believe I used to ride it 20,000 miles a year . Thunderheads made the Iron head a real performer 25 years ago, probably not so hot by todays standards Still runs like bandit starts with one KICK!